Just how badly are we overfishing the oceans?

Humans now have the technology to find and catch every last fish on the planet. Trawl nets, drift nets, longlines, GPS, sonar... As a result, fishing operations have expanded to virtually all corners of the ocean over the past century.

That, in turn, has put a strain on fish populations. The world's marine fisheries peaked in the 1990s, when the global catch was higher than it is today.* And the populations of key commercial species like bluefin tuna and cod have dwindled, in some cases falling more than 90 percent.

So just how badly are we overfishing the oceans? Are fish populations going to keep shrinking each year — or could they recover? Those are surprisingly contentious questions, and there seem to be a couple of schools of thought here.

The pessimistic view, famously expressed by fisheries expert Daniel Pauly, is that we may be facing "The End of Fish." One especially dire 2006 study in Science warned that many commercial ocean fish stocks were on pace to “collapse” by mid-century — at which point they would produce less than 10 percent of their peak catch. Then it's time to eat jellyfish.

Other experts have countered that this view is far too alarmist.** A number of countries have worked hard to improve their fisheries management over the years, including Iceland, Australia, New Zealand, and the United States. "The U.S. is actually a big success story in rebuilding fish stocks," Ray Hilborn, a marine biologist at the University of Washington, told me last year. Overfishing isn't inevitable. We can fix it.

Both sides make valid points — but the gloomy view is hard to dismiss. That's the argument of a new paper in Marine Pollution Bulletin by Tony Pitcher and William Cheung of the University of British Columbia that weighs in on this broader debate. They conclude that some fisheries around the world are indeed improving, though these appear to be a minority for now.

"Several deeper analyses of the status of the majority of world fisheries confirm the previous dismal picture," they conclude. "Serious depletions are the norm world-wide, management quality is poor, catch per effort is still declining."

The decline of fisheries

One reason the debate about overfishing is so contentious is that it's hard to get a precise read on the state of the world's marine fisheries. (The U.N. Food and Agriculture Organization tries its best in this annual report.) Ideally, we'd have in-depth stock assessments for the entire world, but those are difficult, expensive, and fairly rare.

So, in their paper, Pitcher and Cheung review a number of recent studies that use indirect measurements instead. For example, they note that recent analyses of fish catches suggest that about 58 percent of the world's fish stocks have now collapsed or are overexploited:

History of the status of world fish stocks from the FAO catch database 1950–2008, using a catch-only algorithm revised to meet earlier objections.

It's important to note that this is only one estimate — and a disputed one at that. A 2011 study in Conservation Biology by Trevor Branch et. al., by contrast, estimated that only 7 to 13 percent of stocks were collapsed and 28 to 33 percent "overexploited."*** Focusing on catches can be a tricky metric for judging the state of fisheries (it can be hard, for instance, to track changes in fishing practices over time that might bias the results).

So the authors consider a variety of other metrics, too. One example: The amount of effort that fishermen have put into catching fish has increased significantly in the past three decades, as measured by engine power and days that fishermen spend at sea. But the amount of fish actually caught has nevertheless stagnated since the 1990s:

"Given the increase in global fishing effort, the lack of increase in global fisheries catch in the last decade and the fact that most productive areas have now been exploited by fisheries," Pitcher and Cheung note, it's quite possible that "global exploited fish stocks are likely to be in a decreasing trend."

Could fisheries recover?

That all said, there are also some reasons for optimism. In 2009, ecologist Boris Worm and his colleagues took a look at more than 350 detailed fish stock assessments and found that many fisheries in North America and Europe were actually recovering. In the United States, annual catch limits and market-based permit programs have helped some fish populations rebound.

The real question is whether these success stories are the exception rather than the rule. Pitcher and Cheung argue that the fish stocks analyzed in that 2009 paper make up just 16 percent of the global catch — and are mostly confined to well-managed fisheries in richer countries.

By contrast, more than 80 percent of the world's fish are caught in the rest of the world, in places like Asia and Africa. While data here is patchier, many of the nations in these regions are far less likely to follow the U.N.'s Code of Conduct for Responsible Fisheries, and evidence suggests that "serious depletions are the norm" here:

Correlation of compliance with the FAO (UN) Code of Conduct for Responsible Fisheries (on a scale of zero to ten) with the UN Human Development Index for 53 countries, representing 95% of the world fish catch.

"It all depends where you look," Pitcher said in an interview. "There are a few places where fisheries are doing better: The U.S., Australia, Canada, Norway. But those are relatively rare. In most places, the evidence suggests that things are getting worse." Given that the United States imports 91 percent of its seafood, that's an important caveat.

In theory, the rest of the world could adopt stricter measures to make their fisheries more sustainable, such as catch limits, careful marine planning, and a crackdown on illicit fishing. Boris Worm and Trevor Branch have suggested that particular attention should be paid to "fishing-conservation hotspots" around the world — regions that depend heavily on fishing livelihoods and have lots of biodiversity but are nonetheless badly managed.

Yet many low-income countries still lack the resources to monitor their fisheries. And even richer nations struggle to enforce the laws they have: In Europe, regulators have consistently set lax fishing quotas — in part due to lobbying from the fishing industry. ("Europe is not one of the places that's doing well," says Pitcher, "with a few exceptions like Norway.") Meanwhile, as climate change and ocean acidification disrupt ecosystems in unpredictable ways, regulating fisheries properly may become even more difficult.

"Attempts to remedy the situation need to be urgent, focused, innovative, and global," the paper concludes. But that's harder than it sounds.

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* Global fish production has still grown, however — because the decline in ocean catches is being supplanted by aquaculture and fish farming. See p. 4 here.

** Actually, in some cases it's the same expert on both sides of the debate. Ecologist Boris Worm was a key co-author of the 2006 Science study predicting a collapse in most commercial fisheries by mid-century. He later co-wrote a more optimistic Science paper in 2009 with Ray Hilborn, focusing on detailed stock assessments in the developed world.

--For those interested in the topic, the U.N.'s report on "The State of World Fisheries and Aquaculture 2012" is worth a browse.

--Here's a longer look at the ways the United States is trying to avert fisheries collapses through careful regulations. Some fisheries are starting to rebound, and the U.S. seafood catch was at a 17-year high in 2012. But progress has been uneven in places.